Paleomagnetic study of Cambrian–Ordovician rocks in the Eastern Precordillera of Argentina: some constraints on the Andean uplift of this block

Abstract

A paleomagnetic study on several carbonatic units (limestones to dolomites) of the Argentine Precordillera, spanning the Middle Cambrian to Arenig, is reported. Sampling was done at 23 sites (140 samples) in five localities of the Eastern Precordillera of San Juan province: Zonda (1), Juan Pobre (2) and La Flecha (3) creeks, Loma Redonda (4) and Cerro Pedernal de los Berros (5). Standard AF and thermal demagnetization indicated two different magnetic behaviors. Samples from localities 1 and 2 showed a single, post-tectonic magnetic component attributed to a Recent viscous or chemical remagnetization. Samples from localities 3 to 5 showed two magnetic components: a low unblocking temperature (350°C) magnetization coincident with that found at the previous localities and a high temperature one (up to 500°C). This second component was also found to be post-folding (post-Devonian?) and its in situ mean direction is Dec: 277.1°, Inc: 69.4, α95: 9.3°, N=10. This is not consistent with any expected direction for the study area between Carboniferous and Recent times. It is therefore inferred that the rocks carrying it underwent substantial tectonic rotation since its recording. Comparison with the magnetization carried by similar Early Ordovician limestones in the Central Precordillera and the San Rafael Block, and assigned to a Permian remagnetizing event, suggests that the same process affected the carbonatic rocks in the Eastern Precordillera and, therefore, the age of the component is likely Permian. Restoration of the in situ mean direction to the expected direction for a Permian remagnetization can be obtained by a 40° ccw rotation around an axis plunging 30° to N30°E. This is compatible with uplifting of the Eastern Precordillera as a series of quasi-rigid blocks along a major E dipping N to NE backthrust during the Andean orogeny.